Blends of polyester fibers and methods of preparing the blends have been widely reported in the prior art. The resulting products frequently are co-polyesters. For example, U.S. Pat. No. 2,965,613 teaches a co-polyester of ethylene terephthalate-ethylene isophthalate in which the ethylene terephthalate units comprise from 90 to 15 percent of the ethylene terephthalate and ethylene isophthalate units in the co-polyester. The co-polyester is prepared by the ester interchange method and comprises linear molecules and contains two kinds of repeating units. As the ratio of the isophthalate units increased, relative to total units, the second order transition temperature of the copolymer decreased as did the softening point, and solubility in solvents increased. U.S. Pat. No. 4,551,368 teaches polyester melt blends which have high gas barrier properties which comprise blends of polyethylene isophthalate with polyethylene terephthalate or polybutylene terephthalate. The resulting melt blends have a lower melting point than either the polyethylene terephthalate or polybutylene terephthalate component of the blend, indicating the melt blend process results in a copolymer. The melt blend process can occur either through reactor blending or blending through an extruder. A third procedure involves merging two reaction melt streams together and mixing them. Conventional temperatures and catalysts are taught as useful during the transesterification of the various polyesters.
As is noted above, the melt blending of polyesters typically concerns a transesterification reaction between the two polyesters and results in a copolymer. As is also well-known, the presence of transesterification inhibitors can inhibit the polymerization reaction. U.S. Pat. No. 4,069,278 teaches that in the poly condensation of ethylene glycol and dimethyl terephthalate catalyzed by calcium acetate and antimony oxide, no phosphorus containing stabilizers (catalyst inhibitors) were added at anytime. Otherwise a blend of polybutylene terephthalate and commercial polyethylene terephthalate containing a catalyst inhibitor would not solid state polymerize to obtain a higher melt viscosity thermoplastic polyester blend using a melt blending procedure.
The utility of melt mixing two modified polyesters to produce a modified polyester which is stable in terms of thermal properties even after being exposed to a high temperature over a long period of time during fabrication is taught in Japanese Patent JP 5,287,067. The addition of an esterification inhibitor to a melt mixing of two polyesters produces a modified polyester wherein the change of thermal properties is very small. The teachings of JP 5,287,067 includes a review of methods to obtain an inhibitory effect or stoppage of the ester-exchange reaction which occurs during melt-mixing two or more different polyesters. Addition of organic or inorganic phosphorus compounds can inhibit the ester-exchange reaction, as will also the addition of a carboxylic acid ester, a carbonate ester, a carbonate compound, and a lactam compound. JP 5,287,067 teaches a preferred ester-exchange inhibitor is a monoisocyanate compound. In a similar procedure, Japanese patent JP 5,287,044 teaches monoepoxy compounds, carbondiimide compounds and oxazoline compounds can be used to inhibit the ester-exchange reaction. The addition of the inhibiting compounds to modify the resulting polyester provides a modified polyester with a high melting point which can be used for the production of fibers, films, resins, etc.
Despite the extensive prior art on the preparation of modified polyester compounds by the inhibited ester-exchange reaction, previous investigators failed to recognize the utility of a polyester composition comprising two modified polyesters, each modified polyester containing an inhibitor to inhibit an ester-exchange reaction.
It has been found that the melt blending of two modified polyesters, each polyester inhibited against an ester-exchange reaction, permits the preparation of tailored compositions with good physical properties for applications in shrink fiber and film, and when the compositions are powdered, the resulting powder can be used as a powdered binder for non-woven fabrics.